A transistor s an element that is utilized extensively in semiconductor devices. There may be millions of transistors on a single integrated circuit (IC). A common type of transistor used in semiconductor device fabrication is a metal oxide semiconductor field effect transistor (MOSFET) with either P or N channel transistors. A complementary MOS (CMOS) devices use both positive and negative channel devices in complementary configurations. N-type metal oxide semiconductors (NMOS) are fabricated within P-type wells within a P-type substrate; P-type metal oxide semiconductors (PMOS) within N-type wells situated within the same P-type substrate.
With the increased density of devices and the combination of various types of circuitry, such as logic and radio frequency processing circuits, noise generated in the ICs becomes intense. Such noise is detrimental in the ICs because the integrity of a signal may be compromised, which causes a loss of data or errors in logic or signal processing. In CMOS structures, noise from other nearby devices would interfere with the circuit function, and substrate noise coupling is also an effect that is of concern because it can adversely affect the operation of various other devices. In this regard, transistors in the CMOS structures often require isolation from each other to prevent disturbance from unwanted noise. In the NMOS, deep N-wells surrounding the P-type wells region may be used to electrically shield the device against possible perturbations of noise from those devices. The P/N junction diodes formed between the deep N-well regions and the P-type substrate prevent current flow and the deep N-wells also act as an electrical potential shield.
However, Since N-type wells naturally reduce the noise disturbance from P-type substrate due to small nature P/N junction diodes, the P-type transistors located in the N-type wells still have higher noise disturbance from the P-type substrate than N-type transistors located in the P-type wells within the deep N-wells. The noise and leak current from substrate will have big impact on the operation of the CMOS.